Variable gain amplifier

Abstract

According to an embodiment, a variable gain amplifier includes a differential transistor pair including a first and second transistor. A variable resistor for setting a gain is connected between electrodes the transistor pair. A first variable capacitor is connected to an electrode of the first transistor, and a second variable capacitor is connected to an electrode of the second transistor. Corresponding to the gain setting set by adjusting the variable resistor, capacitance values of the variable capacitors can be adjusted to provide improved frequency characteristics of the variable gain amplifier.

What is claimed is: 
     
       1. A variable gain amplifier, comprising:
 a differential transistor pair including a first transistor and a second transistor, the first and second transistors each having a first electrode, a second electrode, and a gate electrode; 
 a variable resistor connected between the first electrode of the first transistor and the first electrode of the second transistor; 
 a first variable capacitor connected to the first electrode of the first transistor; 
 a second variable capacitor connected to the first electrode of the second transistor; 
 a third variable capacitor connected to the second electrode of the first transistor; and 
 a fourth variable capacitor connected to the second electrode of the second transistor. 
 
     
     
       2. A variable gain amplifier, comprising:
 a differential transistor pair including a first transistor and a second transistor, the first and second transistors each having a first electrode, a second electrode, and a gate electrode; 
 a variable resistor connected between the first electrode of the first transistor and the first electrode of the second transistor; 
 a first variable capacitor connected to the first electrode of the first transistor; 
 a second variable capacitor connected to the first electrode of the second transistor; and 
 a third variable capacitor connected between the second electrode of the first transistor and the second electrode of the second transistor. 
 
     
     
       3. The variable gain amplifier according to  claim 2 , wherein the third variable capacitor comprises a plurality of capacitance sections that are connected in parallel, the capacitance sections each including a first capacitor and a second capacitor connected in series with a switching element, the first capacitor connected between the first transistor and the switching element, and the second capacitor connected between the second transistor and the switching element. 
     
     
       4. A variable gain amplifier, comprising:
 a differential transistor pair including a first transistor and a second transistor, the first and second transistors each having a first electrode, a second electrode, and a gate electrode; 
 a variable resistor connected between the first electrode of the first transistor and the first electrode of the second transistor; 
 a first variable capacitor connected to the first electrode of the first transistor; 
 a second variable capacitor connected to the first electrode of the second transistor; 
 a third variable capacitor connected to the second electrode of the first transistor; and 
 a fourth variable capacitor connected to the second electrode of the second transistor, wherein 
 the first through fourth variable capacitors have capacitance values that cause a polar frequency and a zero point frequency to be equal to each other for a resistance value of the variable resistor that controls a gain setting. 
 
     
     
       5. The variable gain amplifier according to  claim 4 , wherein the differential transistor pair, the variable resistor, and the first through fourth variable capacitors are components of an integrated circuit fabricated with a complementary metal-oxide-semiconductor (CMOS) technology. 
     
     
       6. The variable gain amplifier according to  claim 5 , wherein the variable resistor comprises a plurality of resistance sections that are connected in parallel, the resistance sections each including a first resistor and a second resistor connected in series with a switching element, the first resistor connected between the first transistor and the switching element, the second resistor connected between the second transistor and the switching element. 
     
     
       7. The variable gain amplifier according to  claim 6 , wherein the switching element is an n-channel metal-oxide-semiconductor (NMOS) transistor. 
     
     
       8. A method of controlling a variable gain amplifier that has a differential transistor pair including a first transistor and a second transistor, the first and second transistors each having a first electrode, a second electrode, and a gate electrode; a variable resistor connected between the first electrode of the first transistor and the first electrode of the second transistor; a first variable capacitor connected to the first electrode of the first transistor; a second variable capacitor connected to the first electrode of the second transistor, a third variable capacitor connected to the second electrode of the first transistor; and a fourth variable capacitor connected to the second electrode of the second transistor, the steps of the method comprising:
 setting a gain amount by setting a resistance value of the variable resistor; 
 setting a capacitance value of at least one of the first through fourth variable capacitor such that a polar frequency of the variable gain amplifier and a zero point frequency of the variable gain amplifier become equal to each other. 
 
     
     
       9. The method of  claim 8 , wherein when the resistance value of the variable resistor is increased, capacitance values of the third variable and the fourth variable capacitor are increased so that the polar frequency and the zero point frequency become equal, and when the resistance value of the variable resistor is decreased, capacitance values of the first variable capacitor and the second variable capacitor are increase so that the polar frequency and the zero point frequency become equal. 
     
     
       10. The method of  claim 8 , wherein the variable resistor comprises a plurality of resistance sections that are connected in parallel, the resistance sections each including a resistor connected in series with a switching element.